The present invention relates to a filter for removing particulates such as carbonaceous materials carried by the exhaust gases of a diesel engine, and more particularly to an ultrasonic burner system for burning particulates trapped by a filter.
In recent years, regulations on emission of exhaust gases from vehicles driven by internal combustion engines, particularly diesel engines, are becoming more and more stringent. With respect to particulates carried by the exhaust gases, there is a trend in some states of the U.S. toward quantitatively restricting their level to approximately 1/3 of the present level. However, the conventional means of modifying the combustion system for an engine cannot adequately cope with such stringent control.
Most efforts to remove particulates from the diesel engine exhaust gases have been directed to trapping them by a filter, usually made of a ceramic material, which is periodically regenerated by burning the particulates, the heat required for combustion being supplied either from an electric heater or combustion of a diesel fuel. Some filters may be coated with a catalyst film to lower the combustion temperature; in such a case the filter may be regenerated by the exhaust gases, without using a special heating system. In the aforementioned catalytic filter system, there are problems in that the production costs of the filter are high, and that the catalyst tends to be deactivated by sulfur compounds contained in the exhaust gases. In addition, the exhaust gases alone as the heat source may not always function effectively depending on the engine conditions. The system using an electric heater has a drawback that the power consumption by the electric heater is high, so that this system is unsuitable for automobiles that use batteries as a power source. Accordingly, to overcome such problems, the system that burns part of diesel fuel to generate heat necessary for the regeneration has been recently attracting attention in the industry.
FIG. 5 illustrates a system disclosed by the Japanese Patent Laid-Open No. 43114/1985, which incorporates a system of burning particulates in accordance with the aforementioned system (3). This system is arranged as follows: A housing 7 incorporating a filter 6 made of a ceramic is connected midway in the exhaust pipe 3 of an engine 2 so as to remove particulate matter carried by the exhaust gases. When the amount of the particulate matter trapped by the filter 6 increases, the fluid resistance of the filter 6 also increases. As a result, pressure differential across the filter rises, which is detected by an exhaust gas pressure sensor 10, and a signal obtained by a pressure/electricity converter 11 is input to a controller 20. Subsequently, a switch 16 is closed on the basis of a determination made by the controller 20 to connect a fuel pump 9 and an ignition device 8 to a power source battery 17. Then, fuel is injected from a fuel tank 12 through a burner 4 provided upstream of the filter 6 by means of a fuel pump 9, and, at the same time, a spark is generated at an ignition plug 5 by the ignition device 8 so as to ignite the burner 4. Thus, the high-temperature combustion gas is supplied to the filter 6 to burn the particulate matter trapped by the filter 6, thereby regenerating the filter 6.
In the above-described conventional system, however, since the high-pressure fuel is injected from the burner 4, the flame reaches the filter 6 without being sufficiently mixed with the air flow, and, as shown by the line B in FIGS. 6(a) and 6(b), as for the radial temperature distribution of the filter trap 6, the temperature of the central portion is abnormally higher than that of the outer peripheral portion. This may produce a sufficient thermal shock that can damage the filter.
To overcome this problem, if the length of a combustion line 19 is set to 10 to 15 times greater than the diameter of the filter, the flame can be mixed well with the carrier gas, and the radial temperature distribution of the filter becomes uniform. However, if the diameter of the filter is set to 10 cm, the length of the combustion line 19 becomes 1 m to 1.5 m, so that there is the problem that such a particulate combustion system cannot be mounted on a vehicle having a limited space.
In the case of the invention disclosed in, for instance, Japanese Utility Model Publication No. 29135/1988 in order to solve such problems, an attempt is made to allow the primary air introduced in a tangential direction and the secondary air introduced rearwardly thereof to swirl in mutually opposite directions, but this arrangement disadvantageously requires a complicated system.